In the 3 years I've been in this 2nd floor apartment of a 2 family house, most of the water in my toilet bowl disappears after 15-20 minutes. A plumber came yesterday and replaced a bad mount flange, installed a new wax ring, but the water still "goes away".
On very windy days I have observed the water level varying a little bit....telling me the vent isn't clogged.
The flapper seals well, I don't ever hear the water refilling the tank.
HELP!

First, put some shoes on! Well, to be honest I'm only a little south of you in VA and I'm barefoot right now even if it is February.

The level of water in your bowl isn't related to water running in from the tank but to the level in the built-in trap inside the ceramic of the bowl. Unless it is exceedingly windy where you are and air pressure is literally sicking the water out of your bowl, then you have a hair-line crack somewhere that is letting the water run down the pipe. I'm surprised that the plumber didn't recommend replacing it when he was there, especially after charging you an arm and a leg and your first born's left nut or something. Replacing it yourself is very doable, but if you ***** and moan to the company enough, maybe they would knock off at least the charge for showing up and do you right on the labor. If the new mounting flange and wax ring didn't fix it, then either the toilet is cracked or never properly fit to begin with. Either way I foresee a brand spanking new piece of porcelain in your immediate future.

My shoes tell me that Virginia boy Marlowe is right. The only other thing that will take the water out of the toilet bowl is a partially backed up sewer main and/or improper venting but that is providing that there is another toilet in use on the system. When the other toilet is flushed, it attempts to breathe through your toilet. Typically, a vent is supposed to equalize the pressure inside your plumbing system. Under high winds or super sucker fart fans the water should not move very much, unless there is unequal pressures in the system. Replace your toilet. If your plumber is a good guy, he is going to charge you minimally because he already pulled and re-installed your old toilet. He could have been putting in a new one for the same labor costs plus the toilet.

Thank you, askaplumberdudefirstguy! You put some thought into your response and I appreciate your professionalism. I guess it is a weakness of mine that people who search here are in need of help. And I can't stop myself from telling what little I may know.

I guess it's a good thing that I'm not the only weak guy out there.

Thanks for the responses. Your input confirms what others have told me as well. Time to replace the crapper!